All cultivated forms of watermelon belong to the polymorphic species Citrullus lanatus. As a crop, watermelons are grown commercially wherever environmental conditions permit the production of an economically viable yield. Cultivated watermelons grow as annual plants with large, broad leaves. The leaves may be lobed or simple and are typically orbicular to triangular-ovate in shape. The flowers are monoecious, producing both male and female flowers. The flesh color of cultivated watermelons is red, yellow, or white with ovate to oblong strongly compressed seeds that may be brown or white in color. The characterization of the mature fruit can vary widely among varieties of watermelon. Fruits may be round to oblong or elliptical with rind colors varying from dark green to yellow and rind patterning varying widely.
Many changes that occurred with the domestication of the watermelon relate to fruit morphology, with a specialization in fruit shape, size, and flavor. Cultivated watermelons can vary from 5 to 45 pounds, depending on the variety. In the United States, watermelon is grown in at least 44 states, but the principal watermelon growing states are Georgia, Florida, Texas, and California. Asia is the largest producer of watermelon, with 83.4% of the world production in 2011. Fresh watermelons are consumed in many forms, generally fresh, sliced, or as an ingredient in prepared foods. Over 80% of watermelons grown in the United States are triploid seedless varieties, preferred for their ease of consumption, increased yield, and premium market value over diploid seeded varieties.
Citrullus lanatus is a member of the family Cucurbitaceae. The Cucurbitaceae is a family of about 90 genera and 700 to 760 species, mostly of the tropics. The family includes melons, pumpkins, squashes, gourds, cucumber, loofah and many weeds. The genus Citrullus, to which the watermelon belongs, consists of 4 species, including C. colocynthis, C. rehmii, and C. ecirrhosus, all of which may be crossed with each other successfully. The watermelon is believed to be native to southern Africa and has been cultivated there for about 4000 years.
Regular, seeded watermelon has eleven pairs of chromosomes (2n=2x=22). There also exist tetraploid varieties, which have 44 chromosomes in their somatic cells (2n=4x=44). Popular “seedless” varieties are triploid, meaning they have 33 chromosomes in their somatic cells (3x=33) and are derived from a cross between male diploid and female tetraploid parents. Triploid plants are unable to produce viable gametes; therefore, when triploid plants are grown in the presence of diploid plants, the triploid plants produce seedless fruit. These seedless varieties may sometimes produce fruit with small, edible white ovules, similar to those in immature cucumbers.
Watermelon is an important and valuable field crop. Watermelon may have a wide variety of rind colorations and patterns, from solid green to yellow, to striped, mottled, or spotted. Generally, these traits are naturally-occurring, and fruit exhibiting such patterning may or may not contain the desired attributes of a high-yield, high-quality watermelon suitable for commercial production. Thus, there is a need to provide a diverse array of rind patterning for commercially viable fruits.